HETEROCYCLIC AROMATIC ANIONS WITH 4N+2-PI-ELECTRONS

Equilibrium acidities in DMSO for several cyclic carboxamides, thiocarboxamides, esters, and sulfones that form anions possessing 4n + 2 electrons have been measured. Aromatic stabilization energies (ASEs) for these anions have been estimated by comparing their pK(HA) values with those of open-chain analogues. The ASEs (kcal/mol) are 8.3 for N-methylindolin-2-one, 15.5 for N-methylindoline-2-thione, 7.1 for 2-oxo-2,3-dihydrobenzo[b]furan, 8.5 for 2-oxo-2,3-dihydrobenzo[b]thiophene, 11.4 for 3-phenyl-2H-thiopyran 1,1-dioxide, and 23 for cyclopentadiene. These values need to be corrected, however, for the effects of cyclization on pK(HA) values, which are about 3 kcal/mol for carboxamides and 5 kcal/mol for esters.